14 

 TABLE 10. Effect of Hydrocarbons on [2- C]-Acetate Metabolism to 



CO- in Sediments 



Sediment samples collected in April, 1980. 

 Mean of 3 replicates expressed as DPM x 10 



DISCUSSION 



The major objectives of this study were to address the fate and 

 effects of hydrocarbons from the AMOCO CADIZ spill in anaerobic sedi- 

 ments. It is first necessary to consider the chemistry of the various 

 intertidal sediments with respect to exposure to oxygen. A great deal 

 has been learned recently due to the application of microelectrodes to 

 the study of oxygen distribution and dynamics in marine subtidal and 

 submerged intertidal sediments (Revsbech, et al, 1980a, b). A variety 

 of sediments (including medium — grained sands) exhibited very narrow 

 oxygenated intervals ranging from 2-10 mm below the sediment water in- 

 terface. Oxygen depletion has been measured to occur above the verti- 

 cal discontinuity of Eh in coastal sediments (Revsbech, et al, 1980a). 

 Thus the Eh profile of a sediment may serve as a conservative estimate 

 for the aerobic/anoxic boundary. Eh profiles then indicate that anoxic 

 conditions were likely below 1 cm in the He Grande oiled site and be- 

 low 2-3 cm in Aber Wrac'h sediment. The real depth of oxygen penetra- 

 tion at any given time is likely to be less. Changes in color from 

 brown to black at about 2 mm in the He Grande oiled sediment may in- 

 dicate an extremely narrow aerobic zone. Similar color changes in Aber 

 Wrac'h at about 2-3 cm may indicate that net oxygen penetration in this 

 sediment is deeper, but oxidants other than oxygen could keep Eh high- 

 er. The Eh profiles were measured on undisturbed sediments and pro- 



182 



